Double-walled drill pipes or string as used in "reverse-flow" or "reverse-circulation" techniques have been in existence for many years. Generally they include a large number of outer pipes sections connected end-to-end. A cutting bit is attached to the lowermost section and an impact or percussion device at the surface applies impact blows to the outer pipes causing the drill bit to chip away at the rock or earth formation. A series of inner pipes, connected end-to-end are concentrically disposed within the series of outer pipes to define an annular fluid flow passage between the inner and outer pipes for forcing fluid such as air and/or water down to the cutting bit and a bore for returning the fluid and rock fragments to the surface.
It is known to support the inner pipe by rigidly securing it to the outer pipe. The difficulty with this practice is that the large loads applied to the outer pipe are inherently transmitted to the inner pipe. Since the inner pipe is not intended or designed to take any significant impact loads, fracture of the inner pipe and consequently breakdown of the system takes place.
With a view to avoiding problems of this type, Canadian Pat. No. 857,170 granted to Becker Drilling (Alberta) Ltd. on Dec. 1, 1970 discloses an arrangement whereby the inner pipe rests upon the cutting bit and is permitted to move axially relative to the outer pipe, means being provided to maintain concentricity as between the inner and outer pipe sections. The difficulty with this arrangement is that if the drill string becomes rather long, as they tend to do, the lowermost pipe members must bear the weight of those above them. Consequently, failure of the lowermost pipes takes place.
Accordingly, the prime object of the present invention is to provide a double-walled pipe arrangement which minimizes the stresses applied to the inner pipe either in the form of impact loads generated directly by the percussion device or indirectly by the weight of other inner pipes.